1. Field of the Invention
The present invention generally relates to the protection of loads supplied by inductive circuits and, more specifically, by an alternator. The present invention more specifically relates to the automobile field and to the protection of the electric equipment of a vehicle. Such equipment is most often supplied by a battery and an alternator. The battery is used to supply the different electric devices when the vehicle is stopped, that is, when the alternator is not in operation. Conversely, when the vehicle motor runs, the circuits are directly supplied by the alternator, at the same time as the battery recharges.
2. Discussion of the Related Art
FIG. 1 very schematically shows, in the form of blocks, the electric circuit of a motor vehicle or of any other like system supplied by an alternator. An alternator 1 (ALT) has its output terminals 2 and 3 connected across a battery 4, one of the terminals (for example, 3) forming the most negative terminal or ground. Voltage Va across terminals 2, 3 of the battery and of the alternator supplies different electric pieces of equipment 5 (APPL1, APPL2, APPL3, . . . , APPLN) of the concerned vehicle.
A risk inherent to the operation of an alternator and to the powering by said alternator of downstream electric circuits is linked to the incidental opening of the inductive circuit when the alternator operates. For example, in the application to motor vehicles, jolts when the vehicle is running are likely to disconnect, even temporarily, a terminal of one of the inductive elements. Such a disconnection opens the magnetic circuit, which results in an overvoltage in Ldi/dt for a relatively long time (generally several hundreds of ms for the case of a motor vehicle). Such a so-called xe2x80x9cload-dumpxe2x80x9d overvoltage reaches values on the order 100 peak volts, which are sufficient to damage the electric circuits downstream of the alternator.
FIG. 2 shows in more detail a conventional example of a self-protected alternator 1 and of devices of protection of circuits 5 against overvoltages.
The alternator may be symbolized by an inductive circuit formed of three inductances L1, L2, and L3 arranged in a Y. The terminals corresponding to the free ends of inductances L1, L2, L3 as well as the terminal corresponding to the midpoint of the Y arrangement are individually connected to the anode of a zener diode DZ1, DZ2, DZ3, and DZ4, respectively. The cathodes of zener diodes DZ1 to DZ4 are interconnected and define positive terminal 2 of the electric circuit of the vehicle (connected to the positive terminal of battery 4). The anodes of diodes DZ1 to DZ4 are also respectively connected to the cathodes of zener diodes DZ5, DZ6, DZ7, and DZ8, having their anodes connected to ground 3. The function of diodes DZ1 to DZ8 is to clip the output voltage of the alternator. Diodes DZ1 to DZ8 are most often zener diodes of several tens of volts (more specifically, on the order of 40 volts). The choice of this value of several tens of volts is linked to automobile construction codes, which require that the battery voltage be able to reach 24 volts. Accordingly, the self-protected alternator must enable supply of the battery by means of a voltage at least equal to 24 volts.
A self-protected alternator 1 such as illustrated in FIG. 2 has the disadvantage of still letting through a 40-volt overvoltage for a relatively long time (several hundreds of ms).
The supply circuits of elements 5 almost systematically include a rectifying diode D1, D2, . . . , DN having its anode connected to terminal 2 and its cathode connected to a first electrode of a capacitor C1, C2, . . . , CN of local storage of the voltage at the level of the concerned circuit. The other electrode of capacitor Ci is grounded. The different circuits 5 draw their supply from the power present at the level of capacitor Ci and are, if necessary, provided with voltage regulators (linear or switched-mode regulators) to supply their internal circuits.
In the past, equipment 5 used to be provided with input regulators formed of passive (resistive) circuits, that could be oversized to withstand such voltages. With the increase of electronic circuits equipping motor vehicles, application circuits 5 are much more sensitive. It is thus necessary to protect the different circuits 5 to prevent them from seeing lasting overvoltages (several hundreds of ms) greater than 12 volts.
For this purpose, a zener diode DZA1, DZA2, . . . , DZAN is conventionally provided at the input of each circuit 5. More specifically and as illustrated in FIG. 2, each zener diode DZAi (i ranging between 1 and N) has its cathode connected to positive terminal 2 of each capacitor Ci and its anode connected to ground 3.
Such protection circuits are efficient. They however have several disadvantages.
A first disadvantage is that, because of the strong power that it is necessary to dissipate in case of an overvoltage due to a load-dump type overload, it is generally necessary to place several zener diodes in parallel to form each element DZAi. Such a solution takes up space.
Another disadvantage is that such protection devices associated with the different application circuits are not integrable.
The present invention aims at providing a novel device of protection of electric circuits supplied by an alternator or the like which overcomes the disadvantages of conventional protection circuits.
The present invention more specifically aims at providing an integrable device of protection against overloads linked to an opening of an inductive supply circuit.
The present invention also aims at providing a solution which is compatible with the supply of an automobile battery by a voltage of several tens of volts, and which is compatible with the self-protection circuits of conventional alternators.
To achieve these and other objects, the present invention provides short-circuiting the supply terminals in case an overvoltage occurs.
A problem which is posed is that some pieces of equipment (application circuits) are then no longer supplied. Indeed, the input capacitors Ci of the different application circuits are generally of small value and do not form sufficient reservoirs for a disappearing of the supply for several hundreds of milliseconds. Now, overloads of dump-load type may occur when the vehicle is running. It is thus particularly dangerous to take the risk of a supply failure of some of these equipments (for example, the steering assistance or the braking booster, etc.).
The UK patent application 2 085 246 discloses an overvoltage protection device wherein a thyristor is used to short the supply voltage when this voltage is above a predetermined threshold. Then, the thyristor is open again after a predetermined time by means of a multivibrator. A drawback of such a solution is that it does not make certain that the load elements are supplied when the protection becomes active. The predetermined time can cause a lack of supply of some load elements and the use of a thyristor can cause a further delay before a retriggering if the input a.c. voltage becomes too low at the end of the predetermined time.
The present invention thus more specifically aims at providing a solution which enables maintaining the supply of downstream-connected devices, even in the presence of an overload due to an opening of inductive circuits of load-dump type, while protecting the downstream devices.
More specifically, the present invention provides a device of protection of at least one circuit supplied by a voltage obtained from at least one inductive element, including a switch for short-circuiting the supply provided by the inductive element; and means for turning on the switch when said supply voltage exceeds a predetermined threshold and for turning it off when it is smaller than said threshold.
According to an embodiment of the present invention, the control means are formed of a comparator, a first input of which receives a reference voltage which is a function of said predetermined threshold, and a second input of which receives a voltage representative of the supply voltage provided by the inductive element.
According to an embodiment of the present invention, the device includes an element for damping the variations of the supply voltage at the comparator input.
According to an embodiment of the present invention, the comparator is supplied, from the supply voltage, by means of a capacitor constitutive of said damping element.
According to an embodiment of the present invention, the reference voltage is provided by a zener diode having its anode connected to ground and having its cathode connected to the first non-inverting input of the comparator.
According to an embodiment of the present invention, the second input of the comparator is connected to the midpoint of a voltage-dividing bridge receiving a voltage representative of the supply voltage.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.